Method in a portable communications and data terminal operating to optimize receipt of both incoming CDPD and AMPS messages

ABSTRACT

A portable radio telephone handset operates as a data transfer terminal as well as an analog cellular telephone subscriber station. Two modes of operation, an analog cellular communication mode and a Cellular Digital Packet Data (CDPD) mode, are available in the handset. The handset distinguishes between paging signals indicating CDPD mode communications and those indicating analog cellular communications. The handset also automatically preempts CDPD communications in favor of analog cellular communications such as those carried out in an AMPS configuration. The handset maintains an active status on a CDPD communication channel during a “sleep mode”, when the handset can carry out AMPS activity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This present application is a divisional application of U.S. patentapplication Ser. No. 08/496,282, filed on Jun. 28, 1995, now U.S. Pat.No. 5,819,184, which in turn is a Continuation-In-Part of U.S. patentapplication Ser. No. 08/487,043, filed Jun. 7, 1995, now U.S. Pat. No.6,334,062.

TECHNICAL FIELD

This invention relates generally to wireless communication devices. Moreparticularly, the invention relates to switching wireless portablesubscriber stations between both data and voice modes.

BACKGROUND OF THE INVENTION

The modern analog cellular system for mobile wireless duplex voicetransmission is called “Advanced Mobile Phone Service” (AMPS). The AMPScellular network uses the FCC assigned carrier frequency range of 800 to900 MHz. Automobile-mounted cellular units transmit voice signals to acellular base station within a given cell using up to one watt of power.Hand-held cellular units using battery power supplies transmit voicesignals to a cellular base station within a given cell using up to onequarter watt of transmission power.

The analog human voice was the signal that the AMPS system was firstdesigned to communicate. The AMPS system was optimized for carrying asmany analog voice signals within a given bandwidth of a channel aspossible. Mobility of the cellular telephone using low power mobileunits, FM modulation, and the higher carrier frequency range (800MHz-900 MHz) is achieved through a cellular arrangement of base stationswhereby a user's signal is handed off to the next cell site as he or shemoves into a different cell area. This cellular handoff can cause atemporary loss in transmission or reception. However, temporarily losinga voice signal is not critical because a user knows when there is asignal loss and the voice information can be retransmitted. However,signal loss, even though temporary, poses special problems fortransmission of digital data. Some other AMPS cellular problems causingloss in voice signals are drops in signal strength, reflections,Rayleigh fading, and cellular dead spots.

The availability of portable computers naturally led to the desire toconduct wireless transmission of digital data from a remote location.Presently, the AMPS voice cellular system is being used to transmitdigital data in the form of circuit-switched cellular data across AMPScarrier channels. Raw (baseband) digital data must be converted so thatit can be transmitted and received across the analog AMPS system. Onedisadvantage to using the AMPS system for data transmission is that anarrow channel bandwidth and errors in transmission limit the baud ratefor transmitting and receiving the digital data. As previously stated,loss of raw digital data may be caused by other problems in the AMPSmobile cellular system.

Heretofore, providing efficient wireless communication of both voice anddata signals in an integrated package has been difficult. Furthermore,it is difficult to integrate the features of AMPS voice transmissionwith applications such as data transmission, electronic mail, duplexpaging, as well as the provision of a circuit-switched cellular datainterface such as a wireless fax-modem, into a single hand-held batteryoperated wireless unit. This has been accomplished in part by theCellular Digital Packet Data (CDPD) system described in the CDPDspecification, Version 1.1, incorporated herein by reference asbackground material.

The CDPD communication system shares the same carrier frequenciesassigned to the AMPS channels as described in Part 405, Version 1.1 ofthe CDPD specification.

The base unit or mobile data base station (MDBS 1, as illustrated inFIG. 1), of a CDPD system utilizes a channel within an AMPS cell toestablish a link and communicate to a user's mobile end system. The MDBSmay use other frequencies outside of AMPS that are made available to itby service providers. The mobile end system (M-ES 2) is a portablecomputer, hand-set or other portable electronic device containing asubscriber communication unit. The MDBS serves as a communication linkbetween the user of the subscriber station M-ES 2 and a serviceprovider's network of wire lines, microwave links, satellite links, AMPScellular links, or other CDPD links (such as mobile data intermediatesystem MD-IS 3 and intermediate systems 4, 5, 6) to convey data toanother mobile end system, computer network, or nonmobile or fixedend-user system (F-ES 7, 8).

The CDPD network is designed to operate as an extension of existingcommunication networks, such as AMPS networks and the Internet network.From the mobile subscriber's perspective, the CDPD network is simply awireless mobile extension of traditional networks. The CDPD networkshares the transmission facilities of existing AMPS networks andprovides a non-intrusive, packet-switched data service that does notimpact AMPS service. In effect, the CDPD network is entirely transparentto the AMPS network, which is “unaware” of the CDPD function.

The CDPD system employs connectionless network services (CLNS) in whichthe network routes each data packet individually based on thedestination address carried in the packet and knowledge of currentnetwork topology. The packetized nature of the data transmissions froman M-ES 2 allows many CDPD users to share a common channel, accessingthe channel only when they have data to send and otherwise leaving itavailable to other CDPD users. The multiple access nature of the systemmakes it possible to provide substantial CDPD coverage to many userssimultaneously with the installation of only one CDPD station in a givensector (transmitting range and area of a standard AMPS base stationtransceiver).

The airlink interface portion of the CDPD network consists of a set ofcells. A cell is defined by the geographical boundaries within the RFtransmission range from a fixed transmission site such as MDBS 1, whichcan be received at acceptable levels of signal strength by mobilesubscribers such as M-ES 2. The transmitter supporting the cell may belocated centrally within the cell, with transmission being carried outvia an omni-directional antenna, or the transmitter located at the edgeof a cell and transmitted via a directional antenna to cover just aportion of the cell. This portion of the second type of cell is referredto as a sector. In typical configurations, the transmitters for severalsectors are co-located. The area served by a set of cells has some areaoverlap so that a roaming mobile end system can maintain continuousservice by switching from one cell to an adjacent cell in a mannerroughly analogous to the standard hand-off in an AMPS system. The twocells are considered to be adjacent if an M-ES can maintain continuousservice by switching from one cell to the other. This switching process,called cell transfer, is done independently of normal AMPS hand-offprocedures.

In FIG. 1, the interface (A) between the mobile end system 2 and theMDBS 1 is an “air interface” constituted by a radio frequency link usingstandard AMPS frequencies. The MDBS 1 is connected to other mobile database stations through a mobile data intermediate system (MD-IS) 3. Anumber of mobile data base stations can be under the control of a singlemobile data intermediate system. The mobile data intermediate systemsare connected to each other through intermediate systems such as 4 and 5in FIG. 1.

The intermediate systems are constituted by at least one node connectedto more than one sub-network (such as intermediate system MD-IS 3). Theintermediate system has a primary role of forwarding data from onesubnetwork to another. The mobile data MD-IS 3 performs data packetrouting based on knowledge of the current location of each mobile endsystem within the range of the mobile data base stations under thecontrol of the MD-IS. The MD-IS is the only network entity that is“aware” of the location of any of the mobile end systems. However, undersome circumstances (as defined by the CDPD specification, Version 1.1incorporated herein as background material), particular mobile data basestations will keep track of behavior of specific subscriber stations. ACDPD-specific Mobile Network Location Protocol (MNLP) is operatedbetween each MD-IS (through the intermediate system) to exchangelocation information regarding the mobile end systems.

The overall CDPD network is controlled by a network management system(NMS) 10 having an interface with at least one mobile data intermediatesystem 3. Using a special protocol, programming instructions can betransmitted from the NMS 10 through the MD-IS 3 to any number of mobiledata base stations under proper conditions.

Such programming instructions can be used to convey useful network datato the MDBS, as well as configuring the operation of an MDBS withrespect to such critical features as maintaining channel queues. The NMSalso controls other CDPD system characteristics such as the timing ofpaging messages to coincide with the nondormant periods of the M-EShand-sets. One advantage of the subject CDPD system is the capability ofproviding operating instructions to mobile data base stations from theNMS 10 through an MD-IS 3, or by a direct connection to the MDBS as isoutlined in the detailed description of the MDBS architecture found inthe CDPD specification, Version 1.1, Parts 402 and 403.

FIG. 2 depicts a comparison between the CDPD network illustrated in FIG.1 and the standard AMPS network. The MDBS 1 is the CDPD equivalent to anAMPS base station 21. Both serve as links to mobile users, 2, 2′, and 2″for the CDPD system and 22, 22′ and 22″ for AMPS users. Both AMPS andCDPD functions can be handled by the same hand-set or end systemequipment. Also, the MDBS 1 is preferably located with the AMPS basestation 21 as will be explained in greater detail later.

The MD-IS 3 which acts as a local controller for the CDPD mobile database stations connected thereto is equivalent to the mobile telephoneswitch office (MTSO) 23 used to control a plurality of AMPS basestations 21, 21′ and 21′. In the AMPS system, the MTSO 23 can beconnected to the various base stations 21, 21′, 21″ by way ofcommunication links, either over dedicated landlines or through a PublicSwitched Telephone Network (PSTN) . Likewise, the connection betweenMD-IS 3 and the various mobile data base stations 1, 1′, 1″ controlledthereby is made in the same manner. However, different signalingprotocols are used than those found in the AMPS system.

In comparison to AMPS, the infra-structure requirements of CDPD are verysmall. The CDPD base station equipment is preferably located at acellular carrier's cell site along side existing AMPS base stationcellular equipment. The multiple access nature of the CDPD system makesit possible to provide substantial CDPD coverage to many userssimultaneously with the installation of only one CDPD radio in a givensector. This multiple access is the result of a mobile end-systemaccessing the CDPD channel only when there is data to be sent.

The AMPS base station and the MDBS can use the same RF equipment if bothare co-located. By contrast, the MTSO of the AMPS system and the MD-ISof the CDPD system do not have to be co-located in order to share-RFlinks.

In the AMPS system, the MTSO 23 has the responsibility of connecting theAMPS base station and the mobile station to another party through a PSTN24. The intermediate system 4 of the CDPD corresponds to the use of thePSTN by the AMPS system. Like the AMPS system, the CDPD system must alsouse the public switch telephone network or another landline network forcompleting calls to remote parties or systems via a phone systemterminal network 28. However, the CDPD system employs a differentprotocol than that used by the AMPS system for completing calls over aPSTN.

The MDBS maintains zero or more (up to the MDBS transmission capability)channel streams across the airlink interface, as directed by the MD-IScontrolling that MDBS. The MDBS instructs all subscriber units to changechannels when necessary such as when an AMPS communication is detectedon the CDPD channel. Each subscriber unit's terminal stream is carriedon one channel stream at a time, normally selected by the mobilesubscriber, preferably based upon data received from the MDBS regardingoptimum channels for CDPD use. The forward and reverse traffic in agiven cell (the terminal stream of the MDBS) is carried on a single DSOtrunk, between the MDBS and the MD-IS. Communication between the MDBSand the MD-IS over the DSO trunk follows standard formats such as T1.

Within the CDPD network, digital data is transmitted between the MDBSand the M-ES using Gaussian Minimum Shift Keying (GMSK) modulation.Transmission from the base station to the subscriber station M-ES arecontinuous. Transmissions from subscriber station M-ES to the MDBS use aburst mode in which subscriber station M-ES only accesses a channel whenit has data to send and the channel is not being used by other mobilesubscriber stations. This allows multiple mobile subscriber stations toshare a single channel, and for data transmission characterized byintermittent transactions of relatively small amounts of data, therebygreatly reducing the connection time compared to that when sendingdigital data over conventional circuit-switched cellular modems.

Unlike the signaling schemes used in conventional cellular modems, whichhave been chosen based on the need to operate within the constraints ofthe existing voice signaling system, the GMSK modulation technique usedfor CDPD communication was explicitly selected with the intent ofobtaining both very high bit transmission rates and very good errorperformance in cellular channels. The fact that the choice of modulationwas not constrained by a pre-existing signal structure allows CDPDsystems to achieve substantially greater instantaneous bit rates at verylow received signal levels when compared to those of conventionalcellular modems. This means that CDPD communication systems will providereliable, high speed data transmission in many areas where signalquality is inadequate for good cellular modem performance. Presently theraw (baseband) digital data being transferred across CDPD includeelectronic mail messages, digital fax data, or digital data representinga network connection such that files may be transferred as if currentlyconnected to a local area network.

The mobile data intermediate system MD-IS 3 handles the routing ofpackets for all visiting mobile end systems in its serving area. Twoservices are performed by the MD-IS: a registration service maintainingan information base of each M-ES currently registered in a particularserving location; and a re-address service, decapsulating forwardedpackets and routing them to the correct cell. The serving MD-IS alsoadministers authentication, authorization and accounting services forthe network support service applications.

A CDPD communication system can operate with dedicated channels setaside from the pool of cellular voice channels and reserved for CDPDuse. In the alternative, in a more typical mode of operation, the CDPDcommunication system can use idle time on channels that may also be usedby AMPS communications. In this second case, the mobile data basestation may perform “RF sniffing” to determine which channels areavailable and to detect the onset of voice traffic on the channelcurrently being used for CDPD communication. If an AMPS cellular unitbegins transmitting on a channel occupied by a CDPD communication, theCDPD unit ceases transmitting on that channel and switches to anotheravailable channel (a process called “channel hopping”) or if no otherchannel is available, ceases transmission until a channel becomesavailable for CDPD use.

Although the CDPD system shares existing AMPS radio frequency channels,as stated above, AMPS calls are given first priority, and they arealways able to preempt the use of any channel being used by CDPD.However, the cellular service provider may opt to dedicate one or morechannels to CDPD usage. In this case, AMPS calls will never attempt topre-empt the channels dedicated to CDPD use.

In a normal operation of the MDBS carrying out channel hopping, the MDBSfunctions the monitor activity on AMPS channels. The MDBS maintains alist of the status (occupied by voice or unused) for each channelavailable for CDPD use at the cell. The MDBS selects a channel for CDPDuse from the unused channels in the list based on a combination ofcriteria (not specified in the CDPD standard) . These could include suchconsiderations as: the likelihood that the channel will be required bythe voice system in the near future; the amount of interference presenton the channel; the amount of interference that the CDPD communicationis likely to cause to other voice users in different cells, or on othersectors; or, other factors. The MDBS transmits a list of all channelsavailable for CDPD use (whether currently occupied by a voicecommunication or not) to the subscriber stations. The MDBS may execute achannel hop before the channel is pre-empted by AMPS communication ifthe MDBS determines that another channel is more suitable. In such acase, the MDBS sends a message to the subscriber stations commandingthem to change to the specific channel selected, and then the MDBSexecutes the hop. This sort of hop is much more orderly and efficientthan an unplanned hop since the subscriber stations do not have tosearch for the next channel.

If the present CDPD channel is preempted by AMPS communication, the MDBSselects another channel from those unused by AMPS communications andimmediately hops to it without informing the subscriber station (anunplanned hop). The subscriber station then determines that the CDPDsignal is no longer present on the current channel and searches theother channels in the list to determine the channel (if any) to whichthe CDPD communication has hopped.

The CDPD system has the capability of easily interfacing with theexisting AMPS system and sharing some front-end equipment with it. Totake advantage of this capability, the MDBS must have the capability ofphysically interfacing with existing AMPS base stations. Consequently,the MDBS should be small, non-obtrusive, and easily accessible withoutinterrupting existing AMPS equipment. The MDBS has to be configured soas to easily be connectable to equipment outside the MDBS normallyshared with the AMPS system. This external equipment found in the AMPSbase station includes an antenna system, RF power amplifiers (inparticular, linear amplifiers can be shared with existing AMPS), RFmulticouplers, power splitters, duplexers, and, optional equipment.Since the MDBS shares the environment of the AMPS base station, the MDBSshould not constitute a substantial additional burden upon such supportsystems as environmental control and maintenance. Thus, the MDBS must becompact and flexible, constituting only those elements necessary forcarrying out the MDBS functions necessary at that cell site.

The use of an effective CDPD system has brought about a problem in thata subscriber station must attempt to monitor for incoming calls on bothCDPD and AMPS communication systems. If the subscriber station adheresto the timing of the CDPD system, it is likely that some incoming AMPScommunications will be ignored, despite the pre-emption given to AMPScommunications over CDPD communications. And while priority can be givento monitoring for AMPS communications, it is probable that CDPDcommunications directed to a subscriber station will be lost despite thefact that the CDPD system can buffer incoming paging signals forsleeping CDPD subscriber stations. Existing CDPD communication systemsand existing AMPS communication systems fail to provide efficientmonitoring of both modes of communications to prevent loss of incomingcalls.

BRIEF SUMMARY OF THE INVENTION

One advantage of the invention resides in facilitating efficientswitching between data communication and voice communication withoutloss of data communication where voice communication has priority.

A further advantage is in operating the wireless subscriber station in amanner minimizing loss of both incoming AMPS and CDPD communications.

Another advantage of the invention is in efficiently performing ahand-off operation of a wireless subscriber station in a CDPDcommunication system without losing incoming CDPD or AMPS calls.

These and other advantages of the invention are achieved by a subscriberstation arranged for communication with an analog cellular voicecommunication system and a CDPD communication system, where thesubscriber station is operated to appear to the CDPD system as if thesubscriber station was in the CDPD mode while the subscriber stationactually operated in the AMPS mode of communication. In effect, thesubscriber station deceives the CDPD system into recognizing that thesubscriber station remains in the CDPD sleep mode to avoidde-registration with the CDPD system when the subscriber stationactually enters the AMPS mode of communication.

In accordance with one aspect of the present invention, a subscriberstation is arranged for communication with a first communication systemand a CDPD communication system, where the CDPD system includes firsttime adjustment means for selecting a first time interval betweenconsecutive CDPD paging messages sent from the CDPD communication systemto the wireless subscriber station. The wireless subscriber stationincludes means for requesting communication on the first communicationsystem and means for requesting communication on the CDPD communicationsystem. The subscriber station also has a second time adjusting meansfor selecting a second time interval starting at a most recent CDPDcommunication and ending when the wireless subscriber station isexpected to enter a CDPD sleep mode. The subscriber station alsoincludes means synchronizing the first and second time intervals todetermine respective CDPD and first communication system operationschedules. The wireless subscriber station then uses means for selectingoperation on the first communication system during the second timeinterval.

As another aspect of the invention, the subscriber station operates toappear to be in the CDPD sleep mode while actually in the AMPS mode byswitching back and forth between the CDPD mode and the AMPS mode timedon the basis of both the AMPS paging cycle and the CDPD TEI notificationcycle.

This operation is facilitated by a wireless subscriber station arrangedfor communication with a first communication system and a CDPDcommunication system where the CDPD communication system includes firsttimer means for measuring a first time interval specifying the timebetween consecutive- CDPD paging messages sent from the CDPDcommunication system to the wireless subscribe station. The CDPDcommunication system also includes second timer means for measuring asecond time interval specifying the time between a CDPD system responseto a polling signal from the subscriber station and expected entry ofthe subscriber station into a CDPD sleep mode. The subscriber stationincludes means for requesting communication on the first communicationsystem and on the CDPD communication system, means for measuring thefirst and second time intervals, and means for synchronizing theduration of the first and second time intervals with the CDPDcommunication system. The wireless subscriber station also includesmeans for determining respective CDPD and first communication operationschedules for the subscriber station based upon the first and secondtime intervals and a paging cycle of the first communication system.Also included are means for selecting operation of one or the other ofthe means for requesting communication based upon the operatingschedules to scan for incoming paging signals on the first communicationsystem while continuing to be registered on the CDPD system.

In accord with a further aspect of the invention, a method forcommunicating between a wireless subscriber station and both an analogcellular voice communication system and a CDPD communication systemprovides a subscriber station arranged to monitor both incoming analogcellular voice communications and incoming CDPD communications. Themethod includes the steps of registering the subscriber station with ananalog cellular voice communication system and CDPD system. The CDPDregistration system includes synchronizing a first time interval betweenthe subscriber station and the CDPD system where the first time intervaldefines when the subscriber station is expected to be on the CDPDchannel. In the next step of the CDPD registration, the subscriberstation is switched from the CDPD channel to an analog cellular voicecommunication control channel to monitor for incoming analog cellularvoice communications directed to the subscriber station. In final step,the subscriber station is switched back to the CDPD channel before theend of the first time interval.

As yet another aspect of the invention, a method for operating awireless subscriber station in a CDPD system includes selecting at thewireless subscriber station a first time interval beginning at thecompletion of the most previous CDPD communication between a subscriberstation and a CDPD system and ending when the subscriber station isexpected to enter a CDPD sleep mode. The wireless subscriber stationsynchronizes with the CDPD communication system so that the subscriberstation, along with the CDPD system, measures a plurality of second timeintervals, wherein the second time intervals selected by the CDPD systemdefine a duration of time allowed to the subscriber station beforeregistration. The subscriber station monitors for incoming pagingsignals on a second communication system channel during the first timeinterval. Then, the subscriber station also monitors for incoming secondcommunication system on for a plurality of second time intervals.Finally, the subscriber station changes modes to monitor for incomingCDPD communications on the CDPD channel before expiration of the last ofthe plurality of second time intervals.

In a still further aspect of the invention, the objects are achieved bya method of effecting handoff of a wireless subscriber station from afirst cell to a second cell of a CDPD communication system is performed.A subscriber station contains a cell transfer database pertaining to thefirst cell, and registers in the second cell of the CDPD system bysending a polling receiver ready (RR) signal to a MDBS of the secondcell. The subscriber station determines a first time interval for acomplete Received Signal Strength Indication (RSSI) scan of the secondcell. The subscriber station divides the first time interval into aplurality of overlapping sequential time slots. The subscriber stationthen alternately scans for CDPD activity and analog cellular voicecommunication activity on alternating time slots for the duration of thefirst time interval. The information derived from this scanning createsa second cell transfer database for the second cell. Once this secondcell transfer database has been obtained, the first cell transferdatabase is discarded.

Yet another aspect of the present invention is directed to a wirelesssubscriber station arranged for communication with a first communicationsystem and a CDPD communication system. The wireless station includesmeans for requesting communication on the first communication system andmeans for requesting CDPD communication. The wireless subscriber stationalso includes means for operating on the first communication systemwhile remaining registered on the CDPD communication system.

An additional aspect of the present invention is directed to a methodfor communicating between a wireless subscriber station and both a firstcommunication system and a CDPD communication system. The methodincludes the steps of registering the wireless subscriber station withthe first communication system and then registering the wireless systemwith the CDPD communication system. In the final step the subscriberstation tunes to a control channel on the first communication systemwhile still presumed by the CDPD communication system to be on the CDPDchannel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a CDPD system.

FIG. 2 is a block diagram correlating the CDPD system to an accompanyingAMPS system.

FIG. 3 is a block diagram of a portable radio telephone handset.

FIG. 4 is a representation of the operating modes available to thehandset supporting the present invention when used in the appropriateCDPD communication system.

FIG. 5 is a flow chart depicting parallel operation for the handset andthe Mobile Data Intermediate System (MD-IS) used to carry out thepresent invention.

FIG. 6 is a time diagram depicting subscriber scanning during the RSSIinterval.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Achievement of the advantages of the present invention is facilitated bythe device of FIG. 3. However, the device of FIG. 3 is not necessary tocarry station having the capacity to operate in both the AMPS and CDPDmodes of communication can be programmed to benefit from the operationof the present invention. Whatever type of handset or subscriber stationis used, it is programmed to take advantage of certain operationsnormally carried out between the handset and the CDPD system. Theseinclude but are not limited to cognizance by the handset of the timing(and synchronization therewith) of the various CDPD system operationssuch as notification messages that incoming data is waiting for aspecific subscriber station. Requests for modification of certainparameters by the subscriber station are also utilized as part of thepresent invention. Modifications are not necessary to the MDBS or theMD-IS of the CDPD communication system.

Depicted in FIG. 3 is a block diagram of the portable communicationterminal handset 100 of the present invention. In most respects thisportable communication terminal is similar to conventional portableradio telephone handsets in that it includes a radio frequency module102 having at least one radio frequency transceiver. The radio frequencytransceiver uses a main antenna 104 for both receiving and transmittingthe various types of signals handled by the portable terminal, such asAMPS data (circuit switched cellular data) communication, AMPS voicecommunication and CDPD communication. A diversity antenna 106 is used asa backup to maintain reception under certain adverse conditions. Atelephone type handset 112 is used to facilitate AMPS voicecommunication.

The portable terminal can also be patched into a local public switchtelephone network (PSTN) by way of a digital-analog access interface(DAA) connected to the radio control processor 108. This processor,along with control processor and modem 109, divides the various controlfunctions of the portable terminal including call setup, high levelprotocol, low level protocol, power adjustment, modem operation and datatransfer between an external host computer. To facilitate ease ofsubscriber use, the host computer can be a personal computer (PC) orpersonal digital assistant (PDA) or other electronic device. Theconnection hardware of the portable terminal is of a standard typenormally used with PC external connectors.

The portable data terminal handset and subscriber station as depicted inFIG. 3 can be configured to permit all the modes of operationillustrated in FIG. 4. The circle designated as 200 in FIG. 4 representsthe menu mode selection by either the operator or programmer of theportable data terminal handset. Either of the two modes (AMPS or CDPD)can be selected by an operator using a key pad on the handset. If datais being entered into the portable terminal (handset) 100 by a hostcomputer, either the mode or the predetermined default setting can beselected as part of that data transfer.

For best results, the illustrated system is normally in a low-power“sleep” mode as indicated at 202. This “sleep” or dormant mode resultsin the least amount of power expenditure. Normally, the “sleep” modewill be interrupted every 10-255 seconds to check for messages such asincoming paging signals. If none are received, the CDPD mode remainsidle as indicated at 204. The CDPD can be rendered active as indicatedat 206 by the receipt of a paging signal, a command from the hostcomputer or the handset user to initiate data transfer in the CDPD mode.The advantage of remaining in the CDPD mode is that the battery is notheavily burdened so that, based on current battery technology, talk timeat full transmission power is greater than 1 hour and standby time whilemonitoring the AMPS control channel is greater than 12 hours.

Sleep mode procedures are used to place the handset in the sleep modereferred to previously. The sleep mode is defined as an optional mode ofoperation that might be requested by a subscriber station M-ES duringthe data link establishment procedure (communication between the mobileend station and mobile data intermediate system). The sleep mode isintended to assist power conservation strategies in the mobile endstation. The general operation of the sleep mode permits an M-ES todisable or power-down its receiver and associated circuitry. This modeis a key advantage of CDPD operation.

The sleep mode procedure operates in the “multiple frame establishedstate”. In this operation, if no frames are exchanged on the data linkconnection between a particular subscriber station M-ES and the MDBSafter a period of time defined by the parameter T203, the data linkconnection may be placed in the Temporary Equipment Identifier (TEI)sleep state for the handset. While in this state, the overall networkwill not attempt to transmit information destined for that M-ES. If,after entering the sleep state, new frames become extant and waitinginitial transmission, the network will broadcast a predetermined messageat periodic intervals. This message contains a list of TEI's for whichchannel data is pending. The mobile end stations are expected to wake upat periodic intervals to determine if data from them is pending, andnotify the network that they are willing to receive the pending data.Normally, the M-ES may exit the sleep state at any time.

Parameter T203 represents the maximum time allowed without frames beingexchanged on the data link connection before the M-ES is expected toenter the CDPD sleep mode. On the user/subscriber (M-ES) side, timing ofthe parameter T203 is started or restarted upon transmission of a datalink layer frame of any type on the reverse channel (from the M-ES tothe MDBS). On the network side, the timing of parameter T203 for aparticular M-ES is started or restarted upon receipt of a data linklayer frame (of any type) on the CDPD channel. If the value of parameterT203 expires, the data link entity will enter the TEI sleep state, andissue an indication of this state from the user's side. The layermanagement entity may take power conserving measures, such as disablingthe subscriber radio receiver or other non-essential portions of itscircuitry.

A second parameter, T204, represents the time intervals at which thenetwork side broadcasts TEI notification of pending data for a sleepingM-ES. A single timing operation for parameter T204 is maintained for achannel stream; all user side management entities discover andsynchronize to particular channel streams T204, via the TEI notificationprocedure described in Section 6.8.8 of Part 403 of the CDPDspecification. The number of frames queued in the maximum time for whichthe network attempts to notify an M-ES in the TEI sleep state isimplementation dependent. The network releases a data link connectionand discards all queued frames for which the TEI sleep notificationprocedures are aborted. The maximum number of attempts to notify an M-ESin the TEI sleep state of pending network transmissions is designatedsystem parameter N204. The network normally aborts a TEI sleepnotification procedure for a TEI which has been included in a numberexpressed as parameter N204 of successive TEI notification messageswithout a response from that M-ES. Consequently, the subject M-ES willbe de-registered with the CDPD system.

A complete description of the aforementioned operation is found inSection 6.8 of Part 403 of the CDPD specification, incorporated hereinby reference as background material. The parallel operation of the M-ESand the MD-IS is depicted in the parallel flow charts of FIG. 5. Bothunits recognize when the last CDPD communication by a particularsubscriber at an M-ES took place. In this respect, both the M-ES and theMD-IS can be synchronized to each other. Using internal clocks, bothunits keep track of the time elapsed since elapse of the most recentCDPD communication between the M-ES and the MD-IS, as indicated at step702. Thus, in operation according to this Part of the CDPDspecification, if no data is sent in either direction over the air linkfor a certain length of time (parameter T203), the M-ES will go into thesleep mode and the network will assume that the M-ES is asleep, asindicated at step 703. Once the M-ES enters the sleep mode anothertiming operation is carried out in both the M-ES and the MD-IS.

The total length of this period is defined by the product of parametersT204 and N204, previously described. If the network has data to deliverto the MES that is believed to be asleep, the network will add the TEIfor that M-ES to a list of sleeping units on a particular channelstream, that have data waiting for them. However, the network will notsend that data (step 704). For each time frame measured by the parameterT204, the network will send a TEI indication for a particular subscriberunit M-ES indicating that there is data waiting for that subscriberstation. Thus, the subscriber station will have to be monitoring theCDPD channel at some time during the time frame defined by T204 in orderto determine if a message is waiting for that subscriber station.

The list of subscriber stations having waiting messages is broadcast toall units on that channel stream periodically in a TEI notificationmessage. The time between such notification is specified by theparameter T204. This parameter determines the length of time the M-ES isexpected to sleep before “waking” for its messages. When the M-ESawakens, it waits until it receives a notification message. If the TEIof that M-ES is on the list, it notifies the network that it is ready toreceive data. If the TEI of a particular M-ES is not on that list, thatM-ES goes back to sleep for another period of time, normally specifiedby the parameter T204. If a consecutive number of notifications(specified by the parameter N204) have been made for a TEI without thesubject M-ES indicating that it is ready to receive data, the networkwill then assume that the M-ES is no longer present on the CDPD systemand discards the data that was pending for that M-ES, as indicated atstep 705. If the particular M-ES is handling normal AMPS communicationfor a greater amount of time than that encompassed by the product ofparameters N204 and T204, then the data that is being held for that M-ESis discarded by the network. Thus, CDPD communication is lost due to thenormal operation of AMPS communication.

Since AMPS mode operation is recognized as having preference over CDPDmode operation, the handset preferably spends most of its timemonitoring for AMPS communication and just enough time in the CDPD modeto pick up indication of messages for a subscriber station and avoidde-registration. Consequently, one mode of operating subscriber stationM-ES according to the present invention involves remaining in the AMPSmode monitoring an AMPS control channel while periodically breaking awayto poll the CDPD network. When breaking away from the AMPS mode, thesubscriber station M-ES sends a polling signal to the CDPD network toevoke a response and determine if there is any data waiting on the CDPDnetwork for transmission to that subscriber station. After listening foran appropriate amount of time (usually T203) after the response for areturn message from the CDPD network, the subscriber station thenswitches modes and tunes back to the AMPS channel. Preferably thisswitch-over occurs before any retransmission of relevant AMPS pages thatmight have been missed while the subscriber station was in the CDPDmode.

The time frame during which the subscriber station must wait aftersending the polling signal from the CDPD network is defined by parameterT203. This is the time that the CDPD network has to respond to thepolling signal from the subscriber station to indicate that data iswaiting for that subscriber station. If the subscriber station missesthe waiting data by going back into the sleep mode, the subscriber unitmay be deregistered from the CDPD network.

Once the M-ES has completed CDPD communication, it may appear to go intothe sleep mode once again and the network may interpret the M-ES asbeing asleep again even if the M-ES functions once again in the AMPSmode. For such operation, different values are appropriate for the T203,T204 and N204 parameters than those values for an M-ES actually usingthe CDPD sleep mode in its intended power conservation function.

In practice, the aforementioned operation performs best if the value forthe parameters T203 is smaller than that used for the CDPD sleep mode.Consequently, it does not take very long for the network to interpretthe M-ES as being in the sleep mode. This is necessary to facilitate aquick response to incoming AMPS communications without losingregistration on the CDPD network. The adjustment of the T203 parameterto accommodate the operation of a subscriber station M-ES isparticularly important in light of the fact that it is common for manyAMPS systems to repeat communication pages on the control channel onceevery 4-10 seconds. Consequently, it is desirable for a subscriberstation to be subjected to a parameter T203 that permits the subscriberstation to change modes and check the AMPS control channels insynchronization with the AMPS pages.

If parameter T203 is sufficiently large, the subscriber station M-ESshifts modes to monitor the AMPS control channels during the duration ofthe T203 time frame. Thus, there is a chance that an indication ofincoming CDPD communication for that subscriber station will be lostduring the sojourn on the AMPS communication channel. Also, there is achance that the AMPS page may be missed during the change in the modesfrom AMPS to CDPD to scan for CDPD messages. Optimum operation requiresa value of the T203 parameter based upon the operation of the AMPSpaging system.

In the conventional operation of the CDPD system, the subscriber stationM-ES cannot unilaterally control its own parameter T203 (the time afterwhich the M-ES will go “to sleep” if no data is sent in either directionover the air link). The subscriber station must request a value of T203from the CDPD system, and that value cannot be less than ten seconds.Also, the parameters T204 and N204 are not normally controlled by theM-ES in a CDPD network. Rather, control of these parameters resideswithin the network, in particular with the mobile data intermediatesystem to which a particular M-ES is currently assigned. Consequently,it is very difficult to coordinate the timing of the subscriber stationso as to effectively monitor the AMPS control channel for a predominantamount of time while switching back to the CDPD channel only to receivecritical paging messages. The necessary coordination of subscriberstation operation to the AMPS paging cycle (repeated every 4-10 seconds)while still adhering to the requirements of the CDPD paging cycle isquite difficult.

This situation can be remedied by the operation of the first embodimentof the present invention. This includes a process of adjusting certainparameters such as T203 to help lead the CDPD network to “believe” thatthe M-ES is asleep, during which time the M-ES enters the AMPS mode(including tuning its radio to another channel, if necessary) andperform AMPS functions. Parameter adjustment is done by the subscriberstation. After the AMPS scanning functions are complete, the M-ESre-enters the CDPD mode and notifies the CDPD network that it is readyto receive any data that had been pending (by virtue of the TEI of thatM-ES being on the list of sleeping stations).

Referring to the arrangement depicted in FIG. 5, the subscriber stationM-ES appears to enter the sleep mode at steps 703(a). However, ratherthan remaining in the CDPD sleep mode, the subscriber station switchesmodes and tunes to an AMPS control channel to monitor AMPS activity. Thefrequency at which the subscriber station M-ES checks for AMPS activityis based upon the frequency at which the AMPS system sends pagingmessages indicative of the desired AMPS communication to the subscriberstations.

Likewise, the parameter T204 can be adjusted to facilitate the mostadvantageous coordination between the CDPD and AMPS paging cycles.However, in order to carry out such coordination, it is necessary thatthe subscriber station synchronize its timing with that of the T204parameter, as well as the T203 parameter. This is a critical attributenecessary for all embodiments of the present invention.

The adjustment of all of the T203, T204 and N204 parameters by thesubscriber station is especially helpful in minimizing the time spentmonitoring the CDPD channel, and maximizing the time spent monitoringthe AMPS control channel. However, allowing these parameters to beadjusted by each subscriber station results in substantial control andcoordination problems. Control of these parameters by the subscriberstation M-ES on a practical basis would most likely be limited tocontrol of the T203 parameter due to the limitations of the subscriberstations and the complexity of multiple T204 and N204 values throughoutthe area of control handled by any MD-IS.

Such control can be carried out by the M-ES, either manually orautomatically. Automatic control can be carried out in response to theamount of AMPS usage experienced by a particular M-ES. Thus, the MD-ISdoes not have to configure its operation based upon generalizedstatistical analysis of AMPS versus CDPD usage of all user stationsassociated with an MD-IS serving the M-ES. Rather, parameter selectionwould be the responsibility of each M-ES. These parameters can be sentin any known manner convenient to the normal exchange of data between amobile station and a base station.

However, the use of different values of T204 for each subscriber stationcould cause an undue control burden on the MD-IS controlling the CDPDsystem, or section thereof. While such control problems can be overcomeby substantial hardware and software additions to the MD-IS controllingthe CDPD system, such modification is not within the scope of thepresent invention. Nor is such operation within the realm of the currentadministrative environment controlling the operation of CDPD systems.

In accordance with Version 1.1 of the CDPD specification, Part 403,parameter T203 is set responsive to a request from the subscriberstation M-ES by the CDPD system as controlled by the wireless dataintermediate system MD-IS. If no request is made by the subscriberstation, the CDPD system sets the T203 value by default to 30 seconds.To facilitate coordination with AMPS communication systems, thesubscriber station requests the value for T203 to be set as low aspossible. This minimum value allowed by the CDPD system is 10 seconds.

However, even this minimum value is too long to easily coordinate withthe paging cycle of most AMPS communication systems. A lower value ofT203 can be designated by a subscriber station. However, this value islimited to 0 seconds, and is indicative of the absence of the CDPD sleepmode. This would be impractical for a mobile subscriber station that isattempting to monitor both AMPS and CDPD paging signals at the sametime.

Further, the parameter T204 is set up by the network and is not subjectto variation by the individual subscriber units. A value between 20 and250 seconds is common. Usually this parameter is approximately 1 minute.In the current CDPD administrative environment, subscriber stations donot make requests to alter the T204 parameter value. The T204 parameteris especially critical for the operation of the present invention sinceeach subscriber station must be synchronized to the T204 timing for theMD-IS controlling that part of the CDPD system. While the CDPDspecification does not require that the same synchronization be used forall cells controlled by particular MD-IS, it is reasonable to expectthat this will be done in most cases in order to avoidde-synchronization with the TEI cycle (defined by the T204 parameter)when a mobile subscriber changes cells. This is critical sincere-synchronization of the TEI cycle of the CDPD system can take between30 and 300 seconds. During this time, a high risk of losing incomingAMPS communications would exist.

De-registration (requiring between 30 and 300 seconds to correct) orde-synchronization with the T204 timing would occur only if data waswaiting for a particular subscriber station and that subscriber stationdid not answer pages for a time period defined by the product of theparameter N204 and the parameter T204. The parameter N204 is notadjustable by.the subscriber stations. However, subscriber stations arepermitted to request variations in this parameter from the CDPD network.Parameter N204 is normally set at a value of five. However, values offour and six are also common.

In a typical communications environment, a subscriber station registerswith a CDPD system and requests a T203 parameter of 10 seconds duringthe exchange of TEI messages and synchronization with the TEI timing (aswell as the T203 timing). This process is described as the data linkparameter exchange in Version 1.1 of the CDPD specification, Part 403,page 53, Table 403-8. In the typical AMPS environment accompanying theCDPD environment, AMPS paging signals are sent every 4--10 seconds. Aconflict between scanning for AMPS paging signals and scanning for CDPDindications results due to the aforementioned paging cycles for each.This is especially critical for the initial T203 time duration since asubscriber station will be de-registered with the CDPD system if it doesnot respond to an existing CDPD page during that time. However, if thesubscriber station remains on the CDPD channel during the entireduration of the key 203 time period, the subscriber station is alsoliable to miss an incoming AMPS paging signal.

In order to avoid the possibility of missing an AMPS communication, thepresent invention switches modes to the AMPS control channel during thetime frame designated by the T203 parameter (after a response to thepolling PR signal sent by the subscriber station). The purpose of thisshift from the CDPD mode to the AMPS mode is to remain away from theAMPS control channel for a total of less than approximately 3-5 secondsto avoid the risk of missing a re-transmission of an AMPS paging signal.

Assuming that a cell transfer has just been completed (in approximatelyone second in accordance with the CDPD Specification, Version 1.1, thepart entitled “Radio Resource Management”), and that the CDPD networktakes approximately 3 seconds to respond to the polling message sent bythe subscriber station which has just completed the cell transfer, thenan effective window of approximately 5 seconds exists for the subscriberstation to change modes and monitor an AMPS control channel. There isthe possibility of CDPD de-registration during the 5-10 seconds when thesubscriber station is tuned to an AMPS control channel. However, thispossibility is considered sufficiently small as to be practical withinthe context of this embodiment of the present invention.

After switching to the CDPD channel for the maximum time possiblewithout missing an incoming AMPS paging message, the subscriber stationwill then switch back to the AMPS control channel (if no incoming datahas been detected for the subscriber station on the CDPD channel afterapproximately 3 seconds) At this point, T204 timing becomes criticalsince de-registration and de-synchronization can occur if the subscriberstation fails to detect a TEI notification message N times in a row(where N is the N204 parameter minus one). De-registration orde-synchronization must be avoided since the operation of registrationand re-synchronization can take anywhere from 30 to 300 seconds.However, a time duration of between 30 and 90 seconds is more likely. Ineither case, substantial chance of losing incoming AMPS communicationexists while the re-synchronization is being carried out.De-synchronization and deregistration will not occur if no data is beingheld for that particular subscriber station even if the subscriberstation has not monitored the CDPD channel for N TEI notificationperiods. Normally, the mobile station could switch back from the AMPSchannel to the CDPD channel 2 seconds or so before the end of the T204time period to wait for the TEI notification message. If there is noindication of data for the subscriber station within approximately 5seconds, the subscriber station will switch back to the AMPS controlchannel.

Because synchronization is so critical in order to carry out the modeswitching of the present invention, a problem exists in that there is noway to be certain that the TEI notification messages will be exactlyT204 seconds apart, since they can be buffered in the Mobile Data BaseStation (MDBS) behind other data that is waiting to go out.Consequently, the operation of this embodiment of the present inventionis predicated on the assumption that the TEI notification will not oftenbe delayed more than 2 seconds from the time they are supposed to goout. Such a limitation will also aid in facilitating the MD-IS MDLPre-transmission timers which operate on the order of 5 seconds.Consequently, this limitation is viable in the operation of the CDPDsystem under the aforementioned administrative environment.

A special situation exists if a cell has more than one active CDPDchannel stream. This is normally not the case. However, as demand forCDPD use grows, the situation will soon become a factor in the operationof any CDPD system, and will create a whole new set of problems. This isnot a factor when a directed hop is performed by the subscriber stationsince the subscriber station is told precisely where to tune for theCDPD channel stream with which the subscriber station is associated.However, when an undirected hop occurs, it is necessary for thesubscriber station to scan the active channels to locate the CDPDchannel. Acquisition of the TEI notification message by the subscriberstation is not a problem since synchronization with the T204 parameteralready occurred when the subscriber station registered with the CDPDsystem. However, the TEI notification message is not necessarily insynchronization with the transmission of the channel streamidentification number. This number is needed by the subscriber stationto determine if the correct channel stream has been located.

If there is no immediate data message for a particular subscriberstation upon receiving the TEI, the subscriber station has the option ofwaiting on the CDPD channel for the channel stream identificationmessage, or switching modes to an AMPS channel to make certain that anAMPS page has not been missed. Normally, the channel streamidentification message is sent once every 5 seconds while a typical AMPSpage is carried out once every 4-10 seconds. Consequently, it may benecessary for the subscriber station to jump from the AMPS channel backto the CDPD channel several times to receive at least one channel streamidentification message. Once such a message is received, the subscriberstation ascertains whether that channel stream is already associatedwith the subscriber station. If not, the subscriber station sends apolled receiver/ready (RR) signal to the CDPD system indicating that thesubscriber station has switched channel streams. If the CDPD system hasbuffered (usually for approximately 2 seconds) any incoming CDPDmessages for that subscriber station on the other channel stream, theyare transferred to the new channel stream sent to the subscriber stationwithin the time frame defined by the parameter T203. Once again, thesubscriber station is faced with the problem of staying on the CDPDchannel for the entire duration of the T203 period, or jumping back tothe AMPS control channel to check for incoming AMPS pages. Sincesynchronization exists with the T203 parameter, the subscriber stationhas the option of jumping to the AMPS channel, as previous described,and returning to the CDPD channel before the end of the T203 period toreceive any incoming CDPD data messages. By performing this operation,the subscriber station runs a small risk of being de-registered on theCDPD system if any incoming data messages are missed during the timethat the subscriber station is monitoring the AMPS control channel. Thepossibility is generally considered small enough to be worth risking forthe sake of maintaining optimal AMPS operating efficiency. If there areno data messages waiting for the subscriber station during the T203period, the absence of the subscriber station from the CDPD channel isunknown to the CDPD system and de-registration will not occur. Foroptimal performance (with respect to monitoring for AMPS communication)if the subscriber station determines that it has changed channel streamswithin the same cell, it should go back to the AMPS control channel fora minimum acceptable period before coming back to the CDPD channel andcompleting the channel hop by sending the polling RR message to the CDPDnetwork.

When a subscriber station determines that a transfer is necessary inaccordance with CDPD specification, Part 405, the subscriber stationwill tune back to the AMPS control channel for at least the intervalbetween AMPS pages before coming back to the CDPD channel and carryingout the cell transfer algorithm. If a subscriber unit is “sleeping” (ormonitoring on an AMPS control channel), there is normally no decisionmaking to be made by the subscriber station with respect to hand-off.This is not a problem with the CDPD system, since there is no reversechannel interference. According to the CDPD specification on page405-25, sleeping units do not run the RSSI scan procedure until they areready to transmit. The only factor involved for a sleeping subscriberstation is that it not move so far away from the cell site that itcannot reliably receive the TEI notification messages. This can beassured by checking block error rate criteria (BLER) while tuning to theCDPD channel to check for the TEI notification.

When a subscriber station completes a cell transfer, it sends a pollingmessage to the CDPD network indicating that the subscriber station haschanged channels. This polling message also indicates to the CDPDnetwork that the subscriber station is “awake”. Consequently, thesubscriber station is supposed to stay “awake” on the CDPD controlchannel for at least T203 seconds after the subscriber station receivesthe CDPD network response to a polling message. Otherwise, if theserving MD-IS has data to deliver to the subscriber station before thesubscriber station is expected to go back to the sleep mode, the MD-ISwill attempt to deliver the waiting data immediately. If the attempt todeliver the subject data to the subscriber station is unsuccessful(because the subscriber station is no longer on the CDPD controlchannel), that subscriber station will be de-registered with the CDPDsystem. Thus, there is a strong motivation for the subscriber station toremain on the CDPD control channel for a minimum of 10 seconds asdefined by the T203 parameter, especially if more than one CDPD channelstream exists for the new cell.

However, this time spent exclusively on the CDPD control channel clearlyinvolves some risk of losing incoming AMPS calls, especially in anenvironment where cell transfers (which require that the aforementionedprocess to be carried out) are relatively frequent. Therefore, thesubscriber station does not attempt to stay on the CDPD channel for 10seconds (the T203 value). Rather, a subscriber station stays on the CDPDchannel as long as it reasonably can before going back to the AMPScontrol channel to scan for AMPS paging signals directed to thesubscriber station. By doing this the subscriber station runs only asmall risk of deregistration on the CDPD network.

In order to allow as much time as possible to determine if data will bedelivered to the subscriber station upon completion of cell transfer,cell transfer should take place as quickly as possible. Thus, it isnecessary to keep the cell transfer data base up to date. Otherwise, thesubscriber station will fall back into the initial acquisition of thisdata due to insufficient adjacent cell information. This is not aproblem with initial registration and synchronization since enough timewill be allowed for the cell transfer data base of the RRME to beconstructed. However, it is a problem with cell transfer.

In accord with another aspect of the invention, upon cell transfer, thesubscriber station alternates between the AMPS control channel and theCDPD control channel in a pattern that covers the entire RSSI scaninterval as quickly as possible to rebuild the cell transfer data basefrom the cell configuration message scanned during the RSSI intervalbefore the previous cell configuration data is deleted from thesubscriber station.

Normally, the RSSI scan interval is between 30 and 120 seconds. The scaninterval must encompass the entire cell configuration message needed toprovide the data base for cell transfer. This information includes anindication of the existence of more than one CDPD data channel stream inuse for a particular cell. Because of the necessity of switching to theAMPS mode periodically to check for AMPS paging messages, it is notfeasible for the subscriber station to remain in the CDPD mode for theentire duration of the RSSI scan interval. Consequently, the subscriberstation must switch back and forth between the CDPD and AMPS channels sothat a plurality of RSSI scan periods are necessary in order to obtainthe entire cell configuration data necessary for effective celltransfer. Because this process may take several minutes, and a movingvehicle containing the subscriber station may require frequent hand-off,it is necessary to retain cell configuration data of the previous cellin order to provide some information if a hand-off is needed before thecell configuration data set for the present cell is obtained by thesubscriber station.

An additional factor is present by virtue of switching back and forthbetween the AMPS mode and the CDPD mode. A finite amount of time wouldbe lost each time a mode switch takes place. The time needed to tunebetween the two channels plus the time needed for a complete maximumsize frame to be received by a subscriber station is indicated as theparameter g. This is approximately 300 ms. As a result of this losttime, the RSSI scan interval would have to be received on a staggeredbasis by the subscriber station over a plurality of RSSI scan periods,as illustrated in the time chart of FIG. 6.

As used in FIG. 6, the parameter n is the minimum time between AMPSpages. The parameter g is the guard time necessary to allow for tuningbetween the two modes. As long as the maximum time between AMPS pages isless than the quantity 2n-g, it will be possible to cover the entireRSSI scanning interval in three passes. In order to do this, the RSSIscanner must be divided into even multiples of 2n. Taking as an examplea value of 4.5 seconds for n, g will be 200 ms and the quantity 2n-gwill equal 8.7 seconds. These are typical values, as is therepresentative 90 second duration for the RSSI interval used in FIG. 6.In the first scanning slot (n) the subscriber station monitors the CDPDchannel to obtain the information depicted in that time slot.Afterwards, the subscriber station switches back to monitor the AMPScontrol channel, for a period of time. After scanning the AMPS controlchannel, the subscriber station scans the CDPD channel during the 4ntime slot and obtains the information regarding the cell configurationdata contained therein. As the RSSI interval is repeated, the subscriberstation takes a third pass in the time slot designated 2n beforereturning to the AMPS channel. After monitoring the AMPS channel, thesubscriber station returns to the CDPD channel during the time slotdesignated 5n to collect the data contained therein. After the RSSIinterval has elapsed the second time, the subscriber station remains onthe AMPS control channel until the time interval 3n, during which thesubscriber station collects the data contained therein. Since the RSSIscan interval is 90 seconds (a representative value typical in many CDPDsystems) these three passes are necessary in order for the subscriberstation to obtain the entire cell configuration message. As a result,approximately 4 minutes are needed for the entire cell configurationmessage to be obtained by the subscriber station.

Because of the possibility of de-registration from the CDPD system dueto extended AMPS use, another feature of the invention provides aprocess of checking for CDPD registration periodically. Representativetime periods vary from 5 minutes to 1 hour. However, other time periodscould be used as deemed appropriate based upon the activity in both theCDPD and AMPS systems. During registration check, the subscriber stationwill remain tuned to the CDPD control channel during the entire durationof the time period defined by the T203 parameter. While this means thatthe subscribe station runs the risk of losing an AMPS communicationduring the time frame defined by the T203 parameter, this is anacceptable risk if conducted only once every 5-60 minutes. The risk ismade more acceptable by the fact that if de-registration from the CDPDsystem occurs for the subject subscriber station, the re-registrationprocess will entail a much greater risk of losing an incoming AMPS calldirected to the subject subscriber station. In the alternative, thesubscriber station can remain tuned to the CDPD channel as long aspossible before switching back to the AMPS control channel in time toreceive an incoming paging signal indicative of incoming AMPScommunication. Thus, the subscriber station does not remain on the CDPDchannel for the entire duration of the T203 time period. However, aspreviously explained, the subscriber station runs only a very small riskof deregistration from the CDPD system, or a loss of the proper CDPDdata stream (if more than 2 CDPD data streams are used in the presentcell) To aid optimal performance of the aforementioned, theconfiguration timer (as described in the CDPD specification, Parts 406and 507) should be set for a long duration, preferably an hour or more.A longer configuration time or duration period will result in fewerchances of losing incoming AMPS calls since the configuration timer willoperate less often.

Although a number of arrangements of the present invention have beenmentioned by way of example, it is not intended that the invention belimited thereto. For example, the present invention can be adapted foruse with a variety of different parameter values and administrativeenvironments. Also, the alternate mode to the CDPD mode need not beAMPS. Rather, other types of data communication can be selected.Accordingly, this invention should be considered to include any and allconfigurations, modifications, variations, combinations or equivalentarrangements falling within the scope of the following claims.

What is claimed is:
 1. A method of carrying out handoff of a wirelesssubscriber station from a first cell to a second cell in a CellularDigital Packet Data (CDPD) system controlled by a mobile dataintermediate system (MD-IS), where said wireless subscriber stationcontains a first cell transfer database pertaining to radio frequencycharacteristics of said first cell, said method comprising steps of: (a)sending a polling receiver ready (RR) signal from said wirelesssubscriber station to a CDPD Mobile Data Base Station (MDBS) in saidsecond cell; (b) determining a first time interval for a completeReceived Signal Strength Indication (RSSI) scan of said second cell bysaid wireless subscriber station; (c) dividing said first time intervalinto a plurality of overlapping time slots; (d) alternately scanningsaid CDPD channel and an Advanced Mobile Phone Service (AMPS) controlchannel during successive time slots for a first duration of said firsttime interval, thereby creating a second cell transfer data basepertaining to said second cell; and (e) deleting said first celltransfer database upon acquisition of said second cell transferdatabase.
 2. The method of claim 1, wherein step (d) comprises ofsub-steps of: (i) repeating said alternating scan of said CDPD and AMPScontrol channels for a second duration of said first time interval wheresaid scan on said CDPD channel is carried in time slots not scanned forCDPD activity in a previous duration of said first time interval; and(ii) repeating sub-step (i) until all said time slots of said first timeinterval had been scanned for CDPD activity.
 3. The method of claim 2,wherein each said time slot has the same time duration.
 4. The method ofclaim 3, wherein each said time slot has a duration defined as 2n, wheren is the minimum time between AMPS pages.
 5. The method of claim 4,wherein a maximum time between AMPS pages is less than a quantity 2n-gwhere g is a time required for moving from one communication mode toanother.
 6. The method of claim 5, wherein all said time slots at saidfirst time interval are scanned within three durations of said firsttime interval.
 7. The method of claim 6, wherein said first timeinterval is divided into a plurality of time slots so that the RSSI scaninterval is an even multiple of 2n.
 8. The method of claim 7, wherein gis approximately 300 ms and the AMPS paging interval is from 4.5 secondsto 8.7 seconds.
 9. The method of claim 2, further comprising the stepsof: (f) selecting a second time interval greater than said first timeinterval; and (g) sending a polling RR from said subscriber station tosaid MD-IS once during each of said second time intervals.
 10. Themethod of claim 2, wherein said second time interval has a duration inthe range of 5 minutes to 1 hour.